The only moment history records Robin Butler behaving with anything less than dignity was in 1991, when he flung himself on the floor alongside John Major. He had fair reason. The IRA had just opened fire on Downing Street.

"We didn't know it was a mortar," he explained later. "The french windows at the end of the cabinet room were burst open by the blast. My first impression was that men were going to rush in with masks on and submachineguns."

These days, dignity has returned. On a hot day last June, all the knights of the garter gathered at Windsor Castle, which was closed to the public, and the master of University College, Lord Butler of Brockwell, was invested by the Queen with his own garter, star, riband, collar and mantle. There was a splendid lunch and, as the college website respectfully records: "Lady Butler even gave a special wave to the Univ contingent as the knights' wives led the ceremony by."

Despite such honours, Robin Butler will be seen by some as a surprising choice to chair such a sensitive and high-profile inquiry as the one announced yesterday into the prewar intelligence on Iraq.

His record as the former head of the civil service shows that he consistently showed deference to those in power. During the height of the Conservative sleaze scandals of the 1990s, Sir Robin, as he then was, chose to believe the dishonest arms sales minister Jonathan Aitken and attacked journalists who were investigating him.

He followed this up by defending Whitehall deceit during the Scott inquiry into covert arms sales to Iraq. During that investigation too, he went out of his way to attack the media for undermining "our system of government" by what he called "grossly distorted and prejudicial allegations".

The picture of this bicycling Old Harrovian that emerged during both these scandals was of a patrician mandarin, protective of Whitehall pieties and resentful of those who sought to puncture them.

The nadir of his career, as he was later ruefully to characterise it, came in 1994, when a triumphalist Jonathan Aitken on the government benches waved a letter from Sir Robin, which he claimed exonerated him: "I hope that the house ... will accept both my assurance and the cabinet secretary's assurance and put an end to the hysterical atmosphere of sleaze journalism by the Guardian."

The letter said he did not regard Mr Aitken as having lied to him. In fact, Mr Aitken, who later went to prison for perjury, had lied comprehensively to the cabinet secretary about his dealings with Arab arms brokers.

Sir Robin had been asked successively to investigate allegations against Mr Aitken and against the equally dishonest then trade minister, Neil Hamilton.

In neither case did he get to the truth. In Mr Aitken's case, the arms sales minister persuaded the then editor of the Guardian, Peter Preston, to refer allegations to Sir Robin before publishing them.

Sir Robin, he said, was "the arbiter of ministerial rules of procedure".

Preston duly sent Sir Robin the documentation, in which he accused Mr Aitken of having his Paris Ritz hotel bill paid by the Saudis at a secret meeting to carve up arms commissions.

The only steps Sir Robin took, it was discovered later, were to ask Mr Aitken if the allegations were true. Mr Aitken denied it. Sir Robin, trustingly, showed Aitken a copy of the dismissive draft letter he proposed to send to the Guardian. In it, Sir Robin wrote that the dispute "seems to be a matter of his word against yours".

Aitken asked Sir Robin to remove this phrase and send a more "minimalist" letter which would discourage the Guardian from publishing the allegations. Sir Robin complied.

The cabinet secretary later went further. When Aitken sent the Guardian a deliberately misleadingly edited copy of a letter from the Ritz, Sir Robin supported him. This led to the dramatic 1994 Commons scene in which Mr Aitken was able to exploit the supportive letter from the cabinet secretary and claim he had been exonerated.

In the midst of the Aitken row, in September 1994, John Major, then prime minister, summoned Sir Robin and asked him also to investigate as yet unpublished allegations of bribe-taking against Neil Hamilton.

Sir Robin delayed for a fortnight before questioning Hamilton, and gave him advance notice of his intentions. Mr Hamilton told an untrue story of the way he had become involved with Mohamed Al Fayed, owner of Harrods and payer of the bribes.

Sir Robin failed to investigate what turned out to be a crucial question - whether Hamilton had any financial relationship with Mr Fayed's lobbyist.

The issue remained unresolved when the allegations against Hamilton exploded into the newspapers, leading eventually to prolonged libel actions, which Hamilton lost, and to the discrediting of the Major government.

The then Sir Robin, who said on his retirement that his great priority had been "trying to make government work", has defended himself against charges that he was a poor detective, by insisting that police work was not his job.

"The only role of officials can be to advise, not to be arbitrators. We are not determinants. We are not this independent arbitration machinery."

He told the Sunday Times: "The Guardian tried to say that I cleared Aitken. I never cleared Aitken. It was for him to defend his own conduct."

But looking back on his role in the sleaze crisis that hit John Major's administration, Lord Butler is well aware that he made mistakes and may be unlikely to want to have the wool pulled over his eyes a second time.

He has told friends he feels anger at those who misled him and regret at his failure to protect the prime minister.

Away from public gaze, Lord Butler has an unofficious, relaxed manner despite his past as one of the most powerful Whitehall mandarins of the past half-century.

He served prime ministers from Ted Heath to Harold Wilson but was most at home with John Major, sharing his love of cricket.

Lord Butler captained the Mandarins XI and was extremely competitive. One senior official recalls: "The joke was, if you ran him out, your career was finished".

Known in Whitehall as "FERB" - his initials - Lord Butler was one of Mr Major's closest working companions in the dying days of his prime ministership. He stayed on to welcome New Labour but never formed as close a relationship with the new regime.

But the past still dogs him. Alongside the Aitken/Hamilton saga, he was also plunged into what he calls "the long nightmare" of the Scott inquiry, which ran from 1992 to 1996. Lord Justice Scott was given the opposite role to that Lord Butler accepted yesterday. Lord Butler must now attempt to find out whether the Whitehall machine exaggerated the threat posed by Saddam Hussein before the war by spreading alarmist intelligence reports.

By contrast Scott, in the wake of a scandal in which an atttempt was made to prosecute British businessmen for selling ammunition factories to Iraq, was asked to discover whether Whitehall in the 1980s had been too helpful towards Saddam by secretly conniving in arms sales.

During the Scott inquiry, Lord Butler achieved notoriety by defending Whitehall doublespeak and secrecy to a sceptical judge: "You have to be selective about the facts," he said, arguing that government had been entitled to mislead about, for example, delicate negotiations with the IRA or impending devaluations of sterling.

"It does not follow that you mislead people. You just do not give the full information ... It is not justified to mislead, but very often one is finding oneself in a position where you have to give an answer that is not the whole truth."

He joined in ministerial attempts to undermine Scott's inquiry, although he directed his fire ostensibly at the media, saying the inquiry should "undo ... the damage that had been unfairly done to our system of government, to the reputation of the civil service and to individuals".

The victims of the arms-to-Iraq allegations, he said, had been "middle-ranking officials who could not have expected to be thrust into the limelight in the way which they have".

Friends say Lord Butler's very conservatism may lead him to be critical. He has recently privately questioned the ambiguous role of John Scarlett, chairman of the joint intelligence committee and his close relationship with political staff in No 10, notably Alastair Campbell.

He has at times stood firm against powerful figures in Downing Street. As cabinet secretary, he blocked Mr Blair's adviser Jonathan Powell from getting the title of principal private secretary, officially preventing him from access to intelligence reports. He was unhappy too, with the orders in council which gave Mr Powell and Mr Campbell the right to order civil servants around. On that occasion, though, and whatever his private unease, he succumbed.

Career Joined Treasury in 1961 finishing as head of civil service. Retired 1998. Made life peer and served in Lords as cross-bencher. Now master of University College, Oxford. President of University Rugby Football Club

Private secretary to: Edward Heath 1972-74, Harold Wilson 1974-75; principal private secretary to Margaret Thatcher 1982-85, second permanent secretary to Treasury 1985-87; secretary of Cabinet, head of home civil service 1988-98; member of royal commission on reform of Lords 1999